Nowadays, a battery pack, e.g., a Lithium-Ion battery pack, including multiple battery cells is widely used in many electrical products, e.g., hybrid electric vehicle and electric vehicle applications. In general, the battery cells degrade gradually and slowly and each battery degrades differently from other. As a result, voltages and states of charge (SOC) of the battery cells may be different from each other after multiple cycles of charging and discharging, and this difference in degradation leads to unbalances between the battery cells.
During a charging process, if the unbalances between the battery cells occur, when a battery management system detects a battery cell having lowest charge is not at full charge, the battery management system may continue to charging the whole battery pack. As a result, the other battery cells having higher charge may be over-charged. During a discharging process, when the battery management system detects a battery cell having a highest charge is not at full discharge, the battery management system may control the whole battery pack to provide power continuously. As a result, the other battery cells having lower charge may be over-discharged. Hence, a battery management system may need to move energy from a cell or group of cells to another cell or group of cells to balance the battery cells.
FIG. 1 shows a block diagram of a conventional battery management system 100. As shown in FIG. 1, a battery pack 102 includes multiple battery cells 102_1-102_M. A transformer in the battery management system 100 includes a primary winding 104 and multiple secondary windings 106_1-106_M having the same number of turns. The primary winding 104 is coupled to a switch 108 in series. Each battery cell 102_K is coupled to a corresponding secondary winding 106_K (K=1, 2, . . . , M).
During a balancing period, when the switch 108 is turned on, a discharging current flows from the battery pack 102 to the primary winding 104. Energy can be accumulated in a magnetic core of the transformer temporarily. When the switch 108 turns off, currents I1, I2, I3, . . . , and IM are induced in the secondary winding 106_1-106_M and flow to the battery cells 102_1-102_M respectively. Thus the energy stored in the magnetic core can be released to the battery cells 102_1-102_M. Since the currents I1, I2, I3, . . . , and IM are reversely proportional to the voltages of the battery cells 102_1-102_M, a battery cell with a minimal voltage can receive most of the energy. Additionally, a battery cell 102_K (1KM) with a maximal voltage can still receive a current IK even though the current IK is relatively small. Thus each battery cell can receive some energy released from the magnetic core, which may decrease the balancing efficiency of the battery management system 100.